1. Field of the Invention
The present invention relates to precision inertial instruments, and more particularly to rotating, mechanically tuned gyroscope and/or accelerometer devices.
2. Prior Art
The present invention comprises apparatus and techniques for substantially eliminating a major source of disturbance and errors in a certain class of inertial sensors by making certain changes and additions to the structure thereof. This allows the practical implementation of inertial grade sensors for producing two axis angular rate information and two axis acceleration information utilizing a single sensitive element. The invention may also be utilized in various prior art instruments to subdue one of the most troublesome sources of error in such instruments; twice spin frequency vibration induced torques.
The prior art patents describing certain types of prior art instruments and apparatus of the general type or class to which the present invention pertains includes U.S. Pat. No. 3,678,764 to H. F. Erdley, et al; U.S. Pat. No. 3,354,726 to W. J. Krupick, et al; U.S. Pat. No. 3,301,073 and U.S. Pat. No. 3,702,568 to E. W. Howe; U.S. Pat. No. 3,832,906 to R. J. G. Craig; U.S. Pat. No. 3,543,301 to D. Barnett; and U.S. Pat. No. 3,700,290 to W. B. Ensinger. Pertinent publications include the article entited "Dynamically Tuned Free Rotor Gyroscope" published in Control Engineering of June, 1964 pages 67-72 (A. W. Howe) and AIAA Paper No. 65-435 delivered at the AIAA meeting in San Francisco, Calif., July 26-29, 1965, the paper entitled "Dynamics of Ideal Suspensions Applied to Rotating Bodies in Space".
As the foregoing prior art discloses, various mechanizations of rotating, mechanically tuned suspension systems have been successfully employed as the fundamental supporting means for precision inertial instruments, principally gyroscopic rotors having two degrees of freedom. These suspension systems are generally made up of one or more tuned gimbal-torsion spring support combinations, which ideally result in a zero value for one of the two dynamic natural frequencies of the rotating system, and a value of slightly lower than twice the rotation frequency for the other natural frequency, the nutation frequency.
Since the desired response of this type of inertial sensor is centered at zero frequency, any nutation frequency motions are not only of no value, but may cause distortion, rectification, and other errors in the low frequency output measurements if not suitably damped. This damping is generally carried out by a combination of mechanical and control system (instrument capture loop or gimbal servo) design methods.
While this type of mechanization of inertial sensor has proven to be useful for a wide variety of applications, certain fundamental performance limitations exist. These include the common inertial sensor sensitivities to acceleration induced error torques (in association with mass unbalance and anisoelasticity conditions) and other low frequency error torques, such as magnetic and windage induced torques, as well as a class of error torques which is primarily associated with tuned suspension system sensors, namely the twice spin frequency rectifications of both angular and translational input motions. This class of error torques is generally of sufficient magnitude to necessitate both the employment of more than one gimbal and associated torsion spring suspension element per instrument and the associated careful balancing and adjustment methods required to insure effective cancellation of the effects.
Because all of the above mechanizations of inertial sensors are limited to two axes of information (centered about zero frequency) a total of two such sensors are required to provide all three axes of angular rate information. In addition, either two or three separate accelerometers are required for most inertial system requirements. Alternatively, three, two-axis gyro sensors, at least two of which are pendulous (built with a specific mass unbalance along the rotation axis) are required to supply sufficient information from which can be derived all three axes of angular rate and linear acceleration.
Another mechanization of the mechanically tuned suspension system exists in which the two natural frequencies of the instrument are adjusted to slightly greater than zero and (ideally) to precisely twice the rotation frequency. By a proper selection of the mechanical parameters of the instrument, including unequal moments of inertia of the rotor about the two principal axes orthogonal to the rotation axis, the response of the instrument centered at twice the rotation frequency is ideally a function of low frequency angular rate inputs only and is not a function of the usually troublesome low frequency torques. However, such a mechanization is sensitive to twice spin frequency angular rate and linear vibrational input rectification. This sensitivity has been the fundamental limitation to the practical exploitation of this gyro concept, in as much as the means for cancellation of these effects which are used for the more conventional mechanization, described previously, do not apply here.